The present invention relates to an electrolytic process for obtaining platinum of high purity from a concentrated hydrochloric acid solution of contaminated platinum.
Platinum used, for example in instruments, thermocouple elements and catalyzers, is contaminated with base and noble metal impurities after a certain time period depending on the nature of the production process concerned. Scrap platinum having a total metal impurity content of up to 5000 ppm is therefore regularly produced.
Prior to further use in many applications, this scrap platinum must be refined so as to provide, for example, platinum of 99.95% purity for instrument platinum or of 99.99% purity for thermocouple elements. In addition, depending on the intended use, specified quantities of certain impurities must be provided.
The refinement of contaminated platinum may occur by multiple precipitation of the platinum as ammonium platinum chloride. EQU Pt -&gt;H.sub.2 [PtCl.sub.6 ] -&gt;(NH.sub.4).sub.2 [PtCl.sub.16 ] -&gt;Pt
This process, however, has the disadvantages of being very labor-intensive and time-consuming and has many opportunities for loss material. Moreover, the operational personnel are subject to a high allergy risk caused by the ammonium platinum chloride.
These disadvantages could be reduced by using the ion exchanger process according to the WP 147 688. Maximum concentrations of base and noble metals of up to 1000 ppm present as impurities in the platinum can be reduced according to this process, in which a single or multiple precipitation of ammonium platinum chloride is required as a further refinement step. The process can be shortened by a combination of solvent extraction and precipitation in the form of ammonium platinum chloride. Both processes, however, disadvantageously require elaborate equipment and control engineering.
Electrolytic processes for refining gold have been known for a long time (Gmelin Au, Syst. No. 62, 1949) and have been continuously developed further (EP 0 253 783).
From British Patent GB-PS 157 785 and German Published Patent Application 594 408, electrolytic platinum refinement processes are known, which partly operate with combinations of chemical and electrolytic process steps (U.S. Pat. No. 3,891,741).
These processes are all very time-consuming and cannot be reproduced in technically acceptable form in all aspects.
U.S. Pat. No. 4,382,845 describes a partial electrolytic separation of palladium from solutions containing an excess of palladium. Separation according to this process, however, is possible only up to the threshold at which platinum and palladium are present in equal quantities. The separation of further base and noble metals is not mentioned in this publication.
To separate platinum and palladium, a cation exchanger membrane is provided in tile electrolysis cell, whose advantages, however, are not apparent, since platinum and palladium can also be separated without a cation exchanger membrane in the concentration ratio specified and the described voltage range. In addition, this process has the same disadvantage as all other processes, since it can only be operated with a maximum concentration of .ltoreq.100 g/l.